1. Technical Field
This application relates generally to diagnostic X-ray medical imaging.
2. Description of the Prior Art
In the radiography, especially when performed with X-rays having the energy increasingly above 50 keV, which is used for medical diagnosing, the Compton scattering of the primary X-rays takes place with increasing probability. The direction of the propagation of secondary radiation, i.e. of scattered radiation, mainly differs from the direction of the propagation of primary X-radiation being emitted from the focal spot on the anode of the X-ray tube.
A scatter-removing radiographic grid placed between the object to be imaged and the cassette with the X-ray film prevents the secondary X-radiation from impinging on the film and thus from impairing the contrast of the X-ray image originating just from the primary X-radiation. The radiographic grid was protected by G. Bucky through U.S. Pat. No. 1,164,987. It is usually made of strips of a strongly absorbing metal as lead, i.e. of radiopaque strips, between which strips of a weakly absorbing substance as aluminum, i.e. radiolucent strips, are inserted. The long axes of the strips are mutually parallel, however, the strips are differently titled around these axes so that prolongated strip plains intersect themselves in a convergent line. During an exposure the radiographic grid oscillates within its plane transversely to its strips to prevent an imaging of the shadows of the radiopaque strips. Representative parameters of such radiographic grid are: 34 strip pairs/cm; the thickness of the aluminum strips is 0.25 mm and of the lead strips 0.05 mm; the ratio of the grid is 10; the grid is stiffened by an aluminum support cover of a thickness of 0.25 mm on its top and bottom; the transmission of the grid is 60% and its focal length, i.e. the distance of the convergent line from the grid, equals 1500 mm. It is a disadvantage of such radiographic grids that their transmission for the primary X-radiation is low and lies between about 60% and 65%; therefore the patient is exposed to a rather high dose of X-radiation. Furthermore, at the passage of the primary X-radiation across aluminum strips and support covers, the low energy X-radiation is absorbed at a higher rate, whereby the contrast of the X-ray image is impaired. Still another disadvantage of the radiographic grids known in the state of the art exists in that the radiopaque strips are thin and therefore rather translucent for the secondary X-radiation.
In the scanning slit radiography, however, the secondary X-radiation is eliminated by an aft slit, which is swept between the object to be imaged and the cassette with the X-ray film in synchrony with a beam defining the fore slit. The images are excellent. However, either the sweep of the fore and aft slit is slow causing an undesiredly long exposure time during which the patient might move or the X-radiation flux density is high, which requires a high X-ray tube loading. When wide body organs or regions, e.g. the chest, are imaged, the width of the aft slit must be increased in order to reduce the exposure time. It is a consequence thereof that the X-ray scatter impinges on the film, which impairs the quality of the image.